The present invention relates to a system and method for accurate RF output power control using digital and analog power measurements. In certain embodiments, this invention relates to such a system and method in the context of a base station (BS) in a cellular telecommunications system.
One type of radio access scheme which has become increasingly popular over the last several years is known as code division multiple access (CDMA). In a CDMA system, since the same frequency band may be shared by multiple users, signals of other users become interference signals which may degrade the communication quality of a particular user. When a base station (BS) in a cellular telecommunications system or network communicates with near and remote mobile stations (MS) at the same time, it receives the transmitted signal from the near mobile station at a high level, whereas the signal from the remote mobile station is at a much lower level. Thus, communications between the base station and the remote mobile station present a problem in that the channel quality is sharply degraded by interference from the near mobile station. This may be referred to as the near-far problem. High levels of interference may also result when too much output power is used by the base station on the downlink (DL); where xe2x80x9cdownlinkxe2x80x9d defines communications from the base station to mobile station(s).
To obtain high levels of performance in a CDMA or other cellular system, it is important to carefully control the transmit power level of both base stations and mobile stations in the system. Transmit power control limits the amount of self interference produced by the system. Moreover, on the downlink (i.e., from the base station to mobile stations), a precise level of transmit power can serve to balance the forward and reverse link handoff boundaries of a base station or a single sector of a multi-sectored base station. Such balancing may help to reduce the size of the handoff regions, increase overall system capacity, and/or improve remote unit performance in the handoff region.
Failure to control transmit power levels in cellular and other wireless communication systems may adversely affect communication quality. In cellular systems, for example, mobile stations (MS) may transmit a minimum signal level that produces a signal to noise ratio that allows acceptable data recovery at the base station. If a signal transmitted by a MS arrives at the BS receiver at a power level that is too low, the bit error rate may be too high to permit high quality communications due to interference from other mobile stations. On the other hand, if the signal transmitted from the MS to the BS is at a power level that is too high when received at the BS, communication with this particular MS is acceptable, but the high level of power may act as interference to other mobile stations. Thus, closed loop power control systems are typically used so that the base station can control the uplink (UL) powers utilized by respective mobile stations through xe2x80x9cupxe2x80x9d and xe2x80x9cdownxe2x80x9d instructions sent on the downlink to respective mobile stations. It is also desirable to control relative output power levels (i.e., DL power levels) used by base stations in a cellular telecommunications network in order to minimize interference.
Unfortunately, conventional base stations do not have the ability to efficiently provide precise or absolute control over their transmit power level (i.e., DL power control). Absolute accuracy typically is not achieved. To do so, it is necessary to compensate for variations in the gain in the various components comprising the transmit chain of the BS. Variations in the gain typically occur over temperature and aging such that a simple calibration procedure does not guarantee a precise level of output transmit power over time. Variations in the gain may be compensated by adjusting the overall gain in the transmit chain so that the actual transmit power of the BS matches a calculated desired transmit power. Conventional base stations are not equipped with systems which can perform this function in an efficient manner and thus from a practical standpoint lack the capability to limit self interference and balance their forward and reverse link handoff boundaries.
A potential solution would be to improve the accuracy of all of the individual components within the transmit chain of the BS. Typically, accuracies of individual components of the system are too small to achieve the overall desired transmit power level accuracy. Moreover, in order to improve the accuracy of these components, undesirable expense and burdens would be required, and each component would have to have an accuracy much higher than the needed overall accuracy.
Another possible solution would be to use a calibrated accurate CW test tone to calibrate the system. Unfortunately, a CW test tone implies that during a calibration cycle, the normal operation of the BS is disturbed. This is undesirable for base stations operating in cellular telecommunications systems.
U.S. Pat. No. 5,715,526 (the disclosure of which is hereby incorporated herein by reference), discloses an apparatus and method for controlling transmission power in a cellular communications system. Unfortunately, the system of the ""526 patent utilizes formulas to determine the desired output power from data in control bits on a channel-by-channel basis. This channel-by-channel sampling, along with the fact that the ""526 transmit power measurements are not frequency selective, is/are undesirable in that added components and/or expense is required. In the ""526 patent, at least one sample is needed for each channel, which results in an undesirably large number of samples and components required for taking all these samples. Moreover, because the ""526 system is not frequency selective and does not have the capability to track all channels together, it requires use of the illustrated power tracking loop in order to implement compensation for RF loss between the coupler and antenna and implement breathing. The breathing technique disclosed in the ""526 patent is thus very complicated, and its application in the analog domain is not always desirable.
In view of the above, it will be apparent to those skilled in the art that there exists a need for an accurate and simplified system and corresponding method for controlling the transmit power level of a base station in a cellular telecommunications system. There also exists a need in the art for a frequency selective RF analog power sensor/detector for use in such a system.
In the context of a wideband CDMA (WCDMA) type base station (BS), radio frequency (RF) gain is accurately controlled for each of multiple carrier frequencies without having to disturb the normal operation of the amplifier chain once installed in the BS. The power of a summed or combined digital baseband user signal is determined and integrated over a specific time interval. The digital baseband signal is upconverted and amplified by a one or more (e.g., a chain of) RF amplifiers. The power of the transmitted RF signal (DL signal transmitted by the BS) is determined in a manner which is frequency selective, and integrated over the same time interval. The difference between the integrated baseband power and the RF transmit power is then determined in order to provide an overall gain value for the RF chain for the specific carrier frequency measured. This gain value is compared with a target gain value to determine a gain adjustment or correction factor. The gain correction factor is applied in the digital domain prior to digital-to-analog conversion (DAC). Accordingly, gain control is continuously performed during normal operation of the BS. Gain determination and application of the resulting gain adjustment factor is/are determined and performed for each carrier frequency being utilized by the WCDMA base station.
Certain embodiments of this invention are advantageous for a number of reasons. The use of a frequency selective RF power sensor at the BS enables the RF power sensor to be reused in combination with a plurality of different baseband power sensors (one baseband power sensor is provided per carrier frequency to be analyzed). Also, baseband power measurements are performed in the digital domain after the plurality of channels for the specific carrier frequency band at issue have been combined or summed and prior to digital to analog conversion (DAC). A single digital-to-analog converter is typically used for each carrier frequency, so as to handle many channels allocated to that frequency. Thus, the number of samples needed to be taken and the number of digital domain measurements required is significantly less than what is needed in the system of the ""526 patent discussed above where the channels are not summed until in the analog domain. Furthermore, certain embodiments of this invention enable gain adjustment and breathing adjustment to be carried out in the digital domain, which are advantageous compared to such adjustments being made in the analog domain.